Exhaust Recirculation Valve (EGR).......it is valve that used to recirculate the exhaust gases back in to the engine air intake manifold....it usually opens for a very short period of time and shut immediately...but over time(during use) carbon builds up in the valve and most of the time the valve is stuck open,, causing huge amounts of exhaust gasses being recirculated back in the air intake manifold...now the engine sucks more exaust gasses( no oxygen) then the fresh air(oxygen rich) from outside...causing incomplete combustion of diesel in the combustion chamber...hence when the vehicle accelerated dense black clouds of diesel can be seen from the back view mirror and simultaneously a drop in power is felt(this used to happen with my LC a lot) ...
Effects of faulty egr valve:-
intake manifold walls gets contaminated by carbon....and so does the intake valves(sealing ability of valves might reduce) .....
Why egr is use?
The EGR (Exhaust Gas Recirculation) valve helps lower the car's exhaust emissions (gases from the tailpipe). It recirculates the portion of the gasses coming out of the engine by putting in back into the intake manifold, where the engine can burn it again, thereby reducing the emissions.